Phosphate rock (apatite) is the primary commercial source of phosphorus. The majority of the world's phosphate production is used to manufacture fertilizers to sustain agricultural production. The quality of phosphorus reserves is declining and the cost of extraction and processing is increasing. Associated heavy metals like cadmium substituting calcium can be present in phosphate rock at high levels requiring separation. Several countries restrict heavy metal levels in fertilizers. For example, in Sweden P fertilizers having cadmium contents above 5 mg Cd/kg P are imposed with a tax. Some European fertilizer producers have switched suppliers importing only raw material that have set cadmium limits.
All water-soluble phosphate salts such as soluble fertilizers are derived from phosphoric acid. Phosphoric acid is produced commercially by either a ‘wet’ or a thermal process. Wet digestion of phosphate rock is the most common process. Thermal processing is energy intensive and therefore expensive. For that reason, quantities of acid produced thermally are much smaller and mainly used for production of industrial phosphates.
Phosphoric acid for fertilizer production is almost solely based on wet digestion of rock phosphate. The process is mainly based on dissolution of apatite with sulfuric acid. After dissolution of the rock, calcium sulfate (gypsum) and phosphoric acid are separated by filtration. To produce merchant-grade phosphoric acid, high acid concentrations are required and water is evaporated. Calcium sulfate exists in a number of different crystal forms depending on the prevailing conditions such as temperature, phosphorus concentration in the slurry, and level of free sulfate. Calcium sulfate is either precipitated as dihydrate (CaSO4.2H2O) or as hemi-hydrate (CaSO4.½H2O). Phosphoric acid produced through this process is characterized by a low purity.
All ammonium phosphate salts are derived from phosphoric acid. Merchant-grade phosphoric acid, having a concentration of about 54% P2O5, is neutralized with ammonia to form either mono-ammonium phosphate (MAP) or di-ammonium phosphate (DAP) by controlling the ammonia to phosphoric acid mole ratio during the neutralization process. Ammonia is used in liquid or gaseous form. Liquid anhydrous ammonia is usually preferred since surplus heat from other systems is necessary for vaporizing liquid ammonia into a gaseous form. The neutralization of merchant-grade phosphoric acid with ammonia is usually performed in several stages using several reaction vessels. The mole ratio of ammonia to phosphoric acid in the pre-reactor/s is normally held at a level which gives the maximum solubility for the slurry (between 1.4 and 1.45 for production of DAP and usually less than 1 for production of MAP). For operation control, the ammonia to phosphoric acid mole ratio is determined by monitoring the pH of the slurry. Excess heat of reaction is removed from the pre-neutralizer/s by adding water to the reactor/s. Evaporation of the water cools the slurry. As the mole ratio of ammonia to phosphoric acid is increased over 1, un-reacted ammonia escapes from the reactor and the gaseous vapors released must be scrubbed with an acid. The slurry from the pre-neutralization reactor/s which usually contain between 16 to 23% water is usually fed into an ammoniator-granulator to complete the addition of ammonia for the desired product. Completion of the neutralization and additional evaporation of water results in solid particles being formed. It is necessary to recover the un-reacted ammonia from the gaseous vapors by scrubbing with an acid. Thereafter, the solid ammonium phosphates are usually dried in a separate reactor to reduce moisture content. Loss of ammonia from the dryer is usually recovered by scrubbing with acid. The solid ammonium phosphates are normally cooled by passing air through a cooling reactor.
For several applications such as fertigation (the application of water-soluble fertilizers in the irrigation water) and foliar fertilization (spraying fertilizers on leaves) there is a need for fully-soluble ammonium phosphates to avoid clogging of the fertigation equipment by non-dissolved solids. Wet-process phosphoric acid contains a substantial amount of impurities such as iron, aluminum, calcium, magnesium, cadmium, etc. which form water-insoluble solids upon neutralization with ammonia and thus fertilizer-grade ammonium phosphates are not completely water-soluble. Therefore, fully-soluble P fertilizers for fertigation purposes must be specially produced from purified phosphoric acid.
The current technology for phosphoric acid purification is based on extraction of impure wet-process phosphoric acid into an organic solvent (ketones, tri-alkyl phosphates, alcohols, etc.) followed by back extraction with water forming a dilute and pure phosphoric acid which is thereafter concentrated by water evaporation. Purified phosphoric acid is thereafter neutralized with ammonia forming fully-soluble ammonium phosphate products according to the procedure described above.
In general, two processes for solvent extraction of phosphoric acid can be identified: a) partial extraction of phosphoric acid from concentrated solutions, and b) complete extraction of phosphoric acid in the presence of other acids or salts.
Partial extraction of phosphoric acid from concentrated phosphoric acid produced by digestion of apatite with sulfuric acid is the most common process. In this process, only part of the phosphoric acid is extracted into an organic phase. The remaining non-extracted phosphoric acid together with metal impurities is used for production of low-grade phosphate salts such as different fertilizers. Any solvent capable of solvating phosphoric acid can be used in this process, both solvents that have a reasonably constant distribution coefficient down to fairly low concentrations such as alcohols, and solvents which show very little extraction capacity for phosphoric acid below a specific threshold concentration, i.e., the distribution coefficient is very sharply concentration dependent such as for ethers, esters and selected ketones.
A different approach is to obtain complete extraction of phosphoric acid in the presence of high concentrations of other acids or salts. The addition of a second acid such as H2SO4 (U.S. Pat. No. 3,573,005) or a salt such as CaCl2 (U.S. Pat. No. 3,304,157) can improve the distribution coefficient (the distribution ratio of solute between the organic and aqueous phases) of phosphoric acid even at fairly low phosphoric acid concentrations. Although the added acid is also extracted by the solvent its proportion in the organic solvent is normally less than that in the feed solution. Suitable solvents are alcohols, trialkyl phosphates such as tributyl phosphate, etc. which show reasonably constant distribution coefficients down to fairly low phosphoric acid concentrations. The method is recommended for extracting phosphoric acid from remaining impure phosphoric acid resulting from the partial extraction process. A main disadvantage of this approach is that the final aqueous phase is rich in the added acid (i.e. sulfuric acid) or salts together with impurities, which might not have a final use.
The disadvantages of the state-of-the art technologies for production of ammonium phosphates are numerous. The phosphoric acid as produced from the gypsum filter is not suitable for direct manufacture of ammonium phosphate salts. The acid must be further concentrated by water evaporation to a suitable phosphoric acid concentration (usually about 54% P2O5). Normally, concentration of phosphoric acid is done in three stages. The weak acid from the filter (28% P2O5) is evaporated to 40% P2O5 in a single stage vacuum evaporator. The acid is then clarified to remove precipitated solids and the clarified acid is then concentrated to 54% P2O5 in two stages. The inter-stage concentration is about 48% P2O5. The 54% P2O5 acid is used for ammonium phosphate production according to the procedure described above.
To concentrate acids through evaporation is a very energy-intensive process. The amount of steam required for concentrating phosphoric acid usually varies between 2.5-5 tons of steam per ton of phosphorus, depending on production conditions. The energy demand for concentration of phosphoric acid is a major production cost. Expensive equipment such as steam distribution systems, evaporators, effluent gas scrubbers, condensation systems, cooling water systems, liquid effluent treatment systems and acid storage facilities are necessary for production of merchant-grade phosphoric acid. Furthermore, additional equipment is needed for the neutralization of phosphoric acid with ammonia in several stages, drying, cooling and scrubbing of ammonia from gaseous vapors. A major disadvantage is that the quality of the ammonium phosphate product is set by the quality of the apatite raw-material. Produced ammonium phosphates of fertilizer grade are generally contaminated with heavy metals such as cadmium and are not fully-soluble and therefore not suitable for use in applications such as fertigation.
Production of completely-soluble ammonium phosphate salts (technical grade) is more complex and requires purification of merchant-grade phosphoric acid by solvent extraction prior to neutralization with ammonia. The energy costs for water evaporation in this process are much higher since the phosphoric acid needs to be concentrated twice: a) the acid must be concentrated prior to solvent extraction, and b) the purified phosphoric acid is dilute and has to be concentrated again by water evaporation. Additional equipment for production of fully-soluble ammonium phosphates includes facilities for pretreatment prior to solvent extraction, liquid-liquid extraction equipment, liquid-liquid stripping equipment and evaporators for concentrating purified acid.
U.S. Pat. No. 3,298,782 describes a process for the purification of wet-process phosphoric acid which consists of a) extracting phosphoric acid from an aqueous phase to an alcohol-amine organic phase, b) separating the alcohol-amine phase from the aqueous phase, and c) recovering purified phosphoric acid from the alcohol-amine phase. The main objective was to recover purified phosphoric acid by back-extraction with water. In the text it is also mentioned that phosphate salts can be recovered from the alcohol-amine phase by reaction with a base. In one of the examples, an aqueous ammonia solution was used to strip the phosphate from the organic phase into an aqueous phase.
U.S. Pat. No. 3,458,282 describes a method for purifying phosphoric acid by utilizing an amine dissolved in an organic diluent (e.g. kerosene) as an extractant phase to remove either certain impurities from phosphoric acid or to extract phosphoric acid from the aqueous phase. When phosphoric acid was extracted with the amine-diluent solvent, the main objective was to obtain purified aqueous phosphoric acid by back-extraction with water, or to obtain an aqueous phosphate salt solution by reaction with an aqueous base. In the patent text it is also mentioned that it may be possible to remove phosphate from the amine by vaporizing off the organic diluent and treating the remaining material with an aqueous solvent or a gas such as ammonia to precipitate phosphate. To vaporize and condense very large quantities of an organic diluent such as kerosene is both costly and complex.
U.S. Pat. No. 3,894,143 describes a process for obtaining crystallized ammonium phosphate of good quality from wet-process phosphoric acid and ammonia. The process consists of a) forming a mixture of aqueous phosphoric acid and acetone in which all components are miscible with water, b) precipitating impurities by addition of ammonia and separating the precipitated impurities to form a purified mixture, c) contacting the purified mixture with ammonia to produce ammonium phosphate crystals and a supernatant liquid, and d) Separating the ammonium phosphate crystals from the supernatant liquid and distilling the supernatant to separate the acetone for recycling. The disadvantages of this method include distillation of large quantities of acetone, limited yield of ammonium phosphates, and production of large quantities of dilute aqueous ammonium phosphate effluents. The process was therefore not applied in the industry.
In the published international patent application WO 2008/115121, a method and an arrangement for phosphorus recovery are disclosed. Phosphorus ions are extracted from solutions by adsorbing phosphorus ions in a scavenger and by releasing the phosphorus ions into an eluate during regeneration of the scavenger. The regeneration is performed by ammonia. Phosphate anions are precipitated in form of tri-ammonium phosphate upon introduction of excess amounts of ammonia. The ammonia remaining in solution after the precipitation of tri-ammonium phosphate is reused for regenerating the scavenger. Unfortunately, tri-ammonium phosphate is unstable at ambient temperature and atmospheric pressure resulting in the decomposition of the crystal accompanied with release of ammonia. Such unstable crystalline solid is not suitable for direct use in agriculture.
There is a need for an improved method for production of fully-soluble ammonium phosphates such as mono-ammonium phosphate (MAP) or di-ammonium phosphate (DAP), in which the costs associated with the concentration of phosphoric acid by evaporation of water are excluded.